Check if a OneToOne relation exists in Django
Question:
Now I’m using django 1.6
I have two models relates with a OneToOneField
.
class A(models.Model):
pass
class B(models.Model):
ref_a = models.OneToOneField(related_name='ref_b', null=True)
First see my code that points out the problem:
a1 = A.objects.create()
a2 = A.objects.create()
b1 = B.objects.create()
b2 = B.objects.create(ref_a=a2)
# then I call:
print(a1.ref_b) # DoesNotExist Exception raised
print(a2.ref_b) # returns b2
print(b1.ref_a) # returns None
print(b2.ref_a) # returns a2
Now the problem is, if I want to check a A
object, to judge whether it exists a B
objects referencing it. How can I do?
The valid way I tried is only to try and catch an exception, but is there any other prettier way?
My effort:
1 – The below code works, but is too ugly!
b = None
try:
b = a.ref_b
except:
pass
2 – I also tried to check the attributes in a, but not working:
b = a.ref_b if hasattr(a, 'ref_b') else None
Do you meet the same problem, friends? Please point me a way, thank you!
Answers:
So you have a least two ways of checking that.
First is to create try/catch block to get attribute, second is to use hasattr
.
class A(models.Model):
def get_B(self):
try:
return self.b
except:
return None
class B(models.Model):
ref_a = models.OneToOneField(related_name='ref_b', null=True)
Please try to avoid bare except:
clauses. It can hide some problems.
The second way is:
class A(models.Model):
def get_B(self):
if(hasattr(self, 'b')):
return self.b
return None
class B(models.Model):
ref_a = models.OneToOneField(related_name='ref_b', null=True)
In both cases you can use it without any exceptions:
a1 = A.objects.create()
a2 = A.objects.create()
b1 = B.objects.create()
b2 = B.objects.create(ref_a=a2)
# then I call:
print(a1.get_b) # No exception raised
print(a2.get_b) # returns b2
print(b1.a) # returns None
print(b2.a) # returns a2
There is no other way, as throwing the exception is default behaviour from Django One to One relationships.
And this is the example of handling it from official documentation.
>>> from django.core.exceptions import ObjectDoesNotExist
>>> try:
>>> p2.restaurant
>>> except ObjectDoesNotExist:
>>> print("There is no restaurant here.")
There is no restaurant here.
Individual model classes provide a more specific exception called DoesNotExist that extends ObjectDoesNotExist. My preference is to write it this way:
b = None
try:
b = a.ref_b
except B.DoesNotExist:
pass
hasattr
works fine with Django1.11 !
You may use getattr
for shorter version:
getattr(self, 'field', default)
In your case
b = getattr(a, 'ref_b', None)
Much cleaner to use custom django Field based on OneToOneField
, this approach which will allow you to use direct access – just a.ref_b
and result would be instance or None
from django.db.models.fields.related import ReverseOneToOneDescriptor
from django.core.exceptions import ObjectDoesNotExist
from django.db import models
class SingleRelatedObjectDescriptorReturnsNone(ReverseOneToOneDescriptor):
def __get__(self, *args, **kwargs):
try:
return super().__get__(*args, **kwargs)
except ObjectDoesNotExist:
return None
class OneToOneOrNoneField(models.OneToOneField):
"""A OneToOneField that returns None if the related object doesn't exist"""
related_accessor_class = SingleRelatedObjectDescriptorReturnsNone
so the example will look like
class A(models.Model):
pass
class B(models.Model):
ref_a = OneToOneOrNoneField(related_name='ref_b', null=True)
Now I’m using django 1.6
I have two models relates with a OneToOneField
.
class A(models.Model):
pass
class B(models.Model):
ref_a = models.OneToOneField(related_name='ref_b', null=True)
First see my code that points out the problem:
a1 = A.objects.create()
a2 = A.objects.create()
b1 = B.objects.create()
b2 = B.objects.create(ref_a=a2)
# then I call:
print(a1.ref_b) # DoesNotExist Exception raised
print(a2.ref_b) # returns b2
print(b1.ref_a) # returns None
print(b2.ref_a) # returns a2
Now the problem is, if I want to check a A
object, to judge whether it exists a B
objects referencing it. How can I do?
The valid way I tried is only to try and catch an exception, but is there any other prettier way?
My effort:
1 – The below code works, but is too ugly!
b = None
try:
b = a.ref_b
except:
pass
2 – I also tried to check the attributes in a, but not working:
b = a.ref_b if hasattr(a, 'ref_b') else None
Do you meet the same problem, friends? Please point me a way, thank you!
So you have a least two ways of checking that.
First is to create try/catch block to get attribute, second is to use hasattr
.
class A(models.Model):
def get_B(self):
try:
return self.b
except:
return None
class B(models.Model):
ref_a = models.OneToOneField(related_name='ref_b', null=True)
Please try to avoid bare except:
clauses. It can hide some problems.
The second way is:
class A(models.Model):
def get_B(self):
if(hasattr(self, 'b')):
return self.b
return None
class B(models.Model):
ref_a = models.OneToOneField(related_name='ref_b', null=True)
In both cases you can use it without any exceptions:
a1 = A.objects.create()
a2 = A.objects.create()
b1 = B.objects.create()
b2 = B.objects.create(ref_a=a2)
# then I call:
print(a1.get_b) # No exception raised
print(a2.get_b) # returns b2
print(b1.a) # returns None
print(b2.a) # returns a2
There is no other way, as throwing the exception is default behaviour from Django One to One relationships.
And this is the example of handling it from official documentation.
>>> from django.core.exceptions import ObjectDoesNotExist
>>> try:
>>> p2.restaurant
>>> except ObjectDoesNotExist:
>>> print("There is no restaurant here.")
There is no restaurant here.
Individual model classes provide a more specific exception called DoesNotExist that extends ObjectDoesNotExist. My preference is to write it this way:
b = None
try:
b = a.ref_b
except B.DoesNotExist:
pass
hasattr
works fine with Django1.11 !
You may use getattr
for shorter version:
getattr(self, 'field', default)
In your case
b = getattr(a, 'ref_b', None)
Much cleaner to use custom django Field based on OneToOneField
, this approach which will allow you to use direct access – just a.ref_b
and result would be instance or None
from django.db.models.fields.related import ReverseOneToOneDescriptor
from django.core.exceptions import ObjectDoesNotExist
from django.db import models
class SingleRelatedObjectDescriptorReturnsNone(ReverseOneToOneDescriptor):
def __get__(self, *args, **kwargs):
try:
return super().__get__(*args, **kwargs)
except ObjectDoesNotExist:
return None
class OneToOneOrNoneField(models.OneToOneField):
"""A OneToOneField that returns None if the related object doesn't exist"""
related_accessor_class = SingleRelatedObjectDescriptorReturnsNone
so the example will look like
class A(models.Model):
pass
class B(models.Model):
ref_a = OneToOneOrNoneField(related_name='ref_b', null=True)